The problem of providing means to permit light to pass into or out of a microwave cavity and yet substantially completely confine microwaves within the cavity, arises in various applications of microwave energy. For example, it is useful for a microwave oven to have a door which is transparent to light so that the interior of the oven is visible with the door closed, and various solutions of the problem of minimizing microwave leakage while permitting light to pass from the interior of the oven have been proposed. For example, U.S. Pat Nos. 4,049,939, 4,206,338 and 4,211,910 disclose various window structures for microwave oven doors which allow the user to monitor the progress of cooking.
These applications of microwave energy for cooking do not present significant problems in shielding since there is need for only enough light to permit the user to monitor the cooking.
There are, however, applications for microwave energy in which it is desirable, or even necessary, for as much light as possible to be emitted from a microwave cavity. For example, there are applications in which microwaves are used to energize a plasma-forming medium and the plasma emits radiation in the ultraviolet which is used as an energy source.
The goal of providing a cavity which is substantially opaque to microwave energy and transparent to light has been achieved to some extent by the use of a metal mesh as part of a wall enclosing the cavity. However, the metal mesh structures which previously have been used which block only a small portion of the radiant energy, such as, for example, 5%, also permit the passage therethrough of a significant amount of microwave energy, for example, about 1% or more of the energy which is generated. Attempts have been made to reduce the amount of microwave energy which escapes from the microwave cavity, while at the same time not significantly reducing the amount of light leaving the cavity. In one such method, screens in series have been used. See, for example, Japanese laid open application No. SHO58-192458 which describes a microwave energized light source in which a portion of the boundary of the microwave cavity includes two parallel mesh surfaces which are separated by a space. While the amount of microwave energy which escapes from the cavity through a combination of two screens is less than that which passes through a single screen, the use of screens in series does not have the beneficial effect that might be expected. First, assuming that the two screens are identical, there is a reduction of only about 50% in the amount of microwaves which escape to the outside over that which escapes when only one screen is used. This is because those microwaves which pass through the first screen are trapped between the screens and have an equal chance of returning into the cavity or exiting through the second screen. Consequently, the second screen blocks the emission of only about 50% of the microwave energy which is in the space between the screens. Secondly, the double screens have the adverse effect of substantially doubling the amount or radiant energy which is blocked.